In recent years, horn antennas have become quite common for use in coupling microwave energy between a waveguide and free space. Typically, such horns are constructed to have a gradual taper to achieve impedance matching between the waveguide and free space so that virtually all of the energy in the waveguide will be propagated between the waveguide and free space without back reflection. A variety of shapes have been developed for such horns depending on the particular requirements of the microwave signals which it is desired to either transmit or receive. For example, one commonly used horn is the so-called pyramidal horn. Generally, a pyramidal horn has a square cross-section which flares outward in two directions. Another horn in common usage is the circular horn, which essentially has a conical shape for its outward flare.
Since the days of early development of such horn antennas, it has become apparent that certain problems exist in using the basic configurations. For example, whether a pyramidal or circular horn is used, the E plane beam width differs from the H plane beam width, and the beam width side lobes are undesirably high. This results in horns having relatively low efficiency. Accordingly, a variety of attempts have been made to overcome these problems by equalizing the E and H plane beam widths while reducing the side lobes to an acceptable level.
One early technique which was found for achieving relatively close matching of the E and H plane beam widths while reducing side lobes was the so-called "diagonal" horn. As discussed by A. W. Love in his article "The Diagonal Horn Antenna," Microwave Journal, Vol. 5, pages 117-122, March 1962, this type of antenna constituted a pyramidal horn having a square cross-section along its length where, unlike conventional pyramidal horns, the mode of propagation in the horn is such that the electrical vector of propagation is parallel to one of the diagonals of the horn. This creates an internal field within the horn which is a superposition of orthogonal TE.sub.01 and TE.sub.10 modes. As shown in the article by Love, this superposition of fields serves to improve equalization of the E and H plane beam widths and reduce side lobes in the principal planes. Further, the beam widths in the intercardinal planes, i.e. the 45.degree. planes, are relatively similar to the beam widths in the principal planes. Also, the horn has good gain characteristics. However, these improvements were brought about at the cost of generating pairs of cross-polarized lobes in the intercardinal planes. This renders such horns unsuitable in situations where a high degree of polarization purity is necessary. Therefore, the use of such diagonal horns has been somewhat limited in the past.
Accordingly, other attempts followed the development of the diagonal horn seeking to equalize the beam widths in the E and H plane. In particular, a number of horns was developed of the so-called multimode type. These horns operate on the principle of developing a submode in the horn for the purpose of shaping the E or H fields to equalize them. This can be accomplished, for example, by the use of an abrupt step in a circular horn between the input circular waveguide and the conical horn portion, as shown in U.S. Pat. No. 3,305,870 to Potter. Similar techniques involve the use of probes at the throat of the waveguide to generate the submodes. However, both of these techniques result in considerable phase dispersion between the submodes and the dominant mode because of the distance between the point of generation of the submode (i.e. the throat of the horn) and the horn output aperture. Since the submodes and the dominant modes travel at different propagation speeds, the relatively long distance creates the considerable phase dispersion.
This problem in multimode horns led to the introduction of techniques for reducing the phase dispersion. For example, in U.S. Pat. No. 3,305,870, a transition section is provided, the length of which is adjusted to achieve phase equalization between the dominant mode and the submodes. However, this is only achieved at the cost of deteriorating the overall horn performance. Also, the horn tends to be somewhat longer than is typically desired.
Because of the problems of generating the submodes at the throat of the horn, other waveguides were sought to generate the desired submodes. For example, it was found that rather than introduce the submodes at the throat of the horn, these submodes could be introduced through angle changes in the taper of the horn along its length. U.S. Pat. No. 3,662,393 to Cohn is one example of this. These angle changes serve to generate TE/TM.sub.12 submodes when a pyramidal horn is used. These submodes taper the E plane aperture to increase the E plane beam width to the size of the H plane beam width (which is decreased slightly by the taper).
Although this technique of the Cohn patent does provide a horn for equalizing the E and H plane beam widths without the problems of cross-polarized lobes in the intercardinal planes, it suffers from the fact that the length and ultimate aperture size of the horn are considerably larger than desired. For example, using such a multiflare horn at typical satellite frequencies, the output aperture has sides approximately 16 inches long to achieve the desired taper of the E plane aperture for increasing the E plane beam width. In using such a horn as the feed horn for a Cassegrain antenna, a large degree of undesired blockage results due to the size of this multiflare horn. Also, typically this type of horn must be manufactured with several flared sections to achieve the desired equalization. This adds both to the length and difficulty of manufacture. Therefore, although this type of multiflare horn does not have the problems of cross-polarized lobes in the intercardinal planes, as found in diagonal horns, it does not have the advantages of good gain and small size either.